Java集合类总结 （四）

PriorityQueue类

优先队列不管你按照什么顺序插入元素，出队列的时候元素都是按顺序输出的。也就是每次调用remove的时候，都返回当前队列中最小的元素。然后队列中的元素不是维持排序状态的，如果你迭代这个优先队列中的元素，会发现他们不是排好序的。

优先队列使用堆数据结果，堆是一种自我调整的二叉树，对树的add与remove操作可以让最小的元素移动到树的根部。

使用优先队列的典型示例是任务调度，每个任务都有一个优先级，任务以随机顺序添加到队列中。每当启动一个新的任务时，都将优先级最高的任务从队列中取出进行处理

PriorityQueue<GregorianCalendar> pq = new PriorityQueue<>();
pq.add(new GregorianCalendar(1906, Calendar.DECEMBER, 9));
pq.add(new GregorianCalendar(1815, Calendar.DECEMBER, 10));
pq.add(new GregorianCalendar(1903, Calendar.DECEMBER, 3));
pq.add(new GregorianCalendar(1910, Calendar.JUNE, 22));
System.out.println("Iterating over elements...");
for (GregorianCalendar date : pq)
{
     System.out.println(date.get(Calendar.YEAR));
}
System.out.println("Removing elements...");
while (!pq.isEmpty())
{
     System.out.println(pq.remove().get(Calendar.YEAR));
}

输出结果：

Iterating over elements...
1815
1906
1903
1910
Removing elements...
1815
1903
1906
1910

Maps类

Java类库提供两种maps实现：HashMap与TreeMap，他们都实现了Map接口。
哈希（hash）函数与比较函数仅应用于map的key上， map的value只是key的一个关联， value不会进行哈希与比较。

使用HashMap存储元素：

Map<String, Employee> staff = new HashMap<>(); // HashMap implements Map
Employee harry = new Employee("Harry Hacker");
staff.put("987-98-9996", harry);
. . .

• Map有三种view：
  • the set of keys
  • the collection of values (which is not a set)
  • the set of key/value pairs.
    The keys and key/value pairs form a set because there can be only one copy of a key in a map

Set<K> keySet()
Collection<K> values()
Set<Map.Entry<K, V>> entrySet()

迭代所有map中的key：

Set<String> keys = map.keySet();
for (String key : keys)
{
    do something with key
}

迭代map中的key和value：

for (Map.Entry<String, Employee> entry : staff.entrySet())
{
   String key = entry.getKey();
   Employee value = entry.getValue();
   do something with key, value
}

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